Declaring and initializing arrays in C

Question

Is there a way to declare first and then initialize an array in C   So far I have been initializing an array like this   int myArray SIZE     1 2 3 4         But I need to do something like this  int myArray SIZE    myArray    1 2 3 4

User · Answer

The OP left out some crucial information from the question and only put it in a comment to an answer.

I need to initialize after declaring, because will be different depending on a condition, I mean something like this int myArray[SIZE]; if(condition1) { myArray{x1, x2, x3, ...} } else if(condition2) { myArray{y1, y2, y3, ...} } . . and so on...

With this in mind, all of the possible arrays will need to be stored into data somewhere anyhow, so no memcpy is needed (or desired), only a pointer and a 2d array are required.

//static global since some compilers build arrays from instruction data
//... notably not const though so they can later be modified if needed
#define SIZE 8
static int myArrays[2][SIZE] = {{0,1,2,3,4,5,6,7},{7,6,5,4,3,2,1,0}};

static inline int *init_myArray(_Bool conditional){
  return myArrays[conditional];
}

// now you can use:
//int *myArray = init_myArray(1 == htons(1)); //any boolean expression

The not-inlined version gives this resulting assembly on x86_64:

init_myArray(bool):
        movzx   eax, dil
        sal     rax, 5
        add     rax, OFFSET FLAT:myArrays
        ret
myArrays:
        .long   0
        .long   1
        .long   2
        .long   3
        .long   4
        .long   5
        .long   6
        .long   7
        .long   7
        .long   6
        .long   5
        .long   4
        .long   3
        .long   2
        .long   1
        .long   0

For additional conditionals/arrays, just change the 2 in myArrays to the desired number and use similar logic to get a pointer to the right array.

User · Answer

It is not possible to assign values to an array all at once after initialization  The best alternative would be to use a loop   for i 0 i lt N i           array i    i      You can hard code and assign values like --array 0    1  and so on   Memcpy can also be used if you have the data stored in an array already

User · Answer

In C99 you can do it using a compound literal in combination with memcpy  memcpy myarray   int      1  2  3  4    sizeof myarray      assuming that the size of the source and the size of the target is the same     In C89 90 you can emulate that by declaring an additional  source  array  const int SOURCE SIZE      1  2  3  4       maybe  static      int myArray SIZE       memcpy myarray  SOURCE  sizeof myarray

User · Answer

This is an addendum to the accepted answer by AndreyT  with Nyan s comment on mismatched array sizes   I disagree with their automatic setting of the fifth element to zero   It should likely be 5 --the number after 1 2 3 4   So I would suggest a wrapper to memcpy   to produce a compile-time error when we try to copy arrays of different sizes    define Memcpy a b  do                         copy arrays                ASSERT sizeof a     sizeof b   amp  amp            a static assert                   sizeof a     sizeof  a    0         no pointers                memcpy  a    b   sizeof  b                  amp  unnecesary                 while  0                                 no return value      This macro will generate a compile-time error if your array is of length 1  Which is perhaps a feature   Because we are using a macro  the C99 compound literal seems to need an extra pair of parentheses   Memcpy myarray    int      1  2  3  4        Here ASSERT   is a  static assert   If you don t already have your own  I use the following on a number of platforms    define CONCAT TOKENS a  b  a    b  define EXPAND THEN CONCAT a b  CONCAT TOKENS a  b   define ASSERT e  enum  EXPAND THEN CONCAT ASSERT line    LINE      1    e    define ASSERTM e m     version of ASSERT   with message          enum EXPAND THEN CONCAT m   ASSERT line    LINE    1    e

User · Answer

Why can t you initialize when you declare   Which C compiler are you using   Does it support C99   If it does support C99  you can declare the variable where you need it and initialize it when you declare it   The only excuse I can think of for not doing that would be because you need to declare it but do an early exit before using it  so the initializer would be wasted   However  I suspect that any such code is not as cleanly organized as it should be and could be written so it was not a problem

User · Answer

There is no such particular way in which you can initialize the array after declaring it once   There are three options only   1   initialize them in different lines    int array SIZE    array 0    1  array 1    2  array 2    3  array 3    4                      But thats not what you want i guess   2   Initialize them using a for or while loop   for  i   0  i  lt  MAX   i           array i    i      This is the BEST WAY by the way to achieve your goal   3   In case your requirement is to initialize the array in one line itself  you have to define at-least an array with initialization  And then copy it to your destination array  but I think that there is no benefit of doing so  in that case you should define and initialize your array in one line itself   And can I ask you why specifically you want to do so

User · Answer

Is there a way to declare first and   then initialize an array in C    There is  but not using the method you described   You can t initialize with a comma separated list  this is only allowed in the declaration  You can however initialize with     myArray 0    1  myArray 1    2        or   for int i   1  i  lt   SIZE  i        myArray i-1    i

User · Answer

No  you can t set them to arbitrary values in one statement  unless done as part of the declaration    You can either do it with code  something like   myArray 0    1  myArray 1    2  myArray 2    27    myArray 99    -7    or  if there s a formula    for  int i   0  i  lt  100  i    myArray i    i   1    The other possibility is to keep around some templates that are set at declaration time and use them to initialise your array  something like   static const int onceArr         0   1   2   3   4      99   static const int twiceArr        0   2   4   6   8     198     int myArray 7     memcpy  myArray  twiceArr  sizeof  myArray      This has the advantage of  most likely  being faster and allows you to create smaller arrays than the templates  I ve used this method in situations where I have to re-initialise an array fast but to a specific state  if the state were all zeros  I would just use memset      You can even localise it to an initialisation function   void initMyArray  int  arr  size t sz        static const int template      2  3  5  7  11  13  17  19  21       9973       memcpy  arr  template  sz       int myArray 100   initMyArray  myArray  sizeof myArray      The static array will  almost certainly  be created at compile time so there will be no run-time cost for that  and the memcpy should be blindingly fast  likely faster than 1 229 assignment statements but very definitely less typing on your part  -

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